1. Field of the Invention
Work conducted at the facilities of the Tennessee Valley Authority, using a low-temperature concentrated acid hydrolysis scheme for the conversion of the cellulosic fraction of corn stover to monomeric sugars and subsequently to ethanol, demonstrated the high cellulosic conversion efficiencies and potential ethanol yields possible with the so-called concentrated acid hydrolysis process. Nevertheless, the concentrated acid hydrolysis process, although technically viable, was considered to be uneconomical because, prior to utilizing the hydrolyzate produced in that process, the acid present in the hydrolyzate had to be removed or separated therefrom. The most effective means for removal of the acid, which was commonly known at that time, was lime precipitation; however, the cost associated with attendant sulfuric acid and calcium oxide consumption was found to be economically unattractive. Also, the additional cost of disposal of the neutralization product, i.e. gypsum, also militated against commercialization of the concentrated acid hydrolysis process (S. R. Nanguneri, "Acid/Sugar Separation Using Polymeric Ion Exchange Resins: A Process Analysis and Design," master's thesis, University of Southern Mississippi, 1989).
Subsequently, work utilizing an agricultural cellulosic residue in a concentrated acid hydrolysis process resulted in the realization that an integrated cellulose conversion process employing a chromatography system for acid recovery and recycle could provide a viable process.
As is well known, electrolytes can be separated from nonelectrolytes in solution therewith using any of a number of chromatographic techniques including: ion exchange, ion exclusion, and ion retardation. Ion exchange systems, in which ions are exchanged between the solute and the resin bed, have found wide application in industry due mostly to the systems ability to handle relatively high flux rates and a plethora of ionic species. However, because ion exchange does take place, regeneration of the resin is required. Ion exchange resins are typically classified as strongly or weakly acidic or strongly or weakly basic. Strongly acidic resins usually contain sulfonic acid groups, whereas weakly acid resins usually contain carboxylic acid groups. Strongly basic resins usually have quaternary ammonium groups while weakly basic resins usually contain polyamine groups.
An ion exchange resin with interchangeable Na.sup.+ ions is said to be in its sodium form. Introducing an electrolyte such as an aqueous solution of H.sub.2 S0.sub.4 to the system results in an exchange of the Na.sup.+ with H.sup.+ ions and convert the resin to its hydrogen form resulting in an elution of Na.sup.+ from the column. The subsequent elution of H.sup.+ ions from the column, commonly known as "breakthrough," indicates that the column resin has been mostly or fully spent. As may be appreciated, prior to the addition of more acid, such spent resin must be regenerated to its sodium form.
Ion exclusion systems, sometimes referred to as electrolyte exclusion systems, employ the same resins used in ion exchange systems, discussed supra, but differ in that the ionic functionality of the resin is the same as that of the electrolyte and, therefore, there is no exchange of ions. As will be appreciated, resins used in the instant invention are typically sulfonated polystyrenes with some degree of divinylbenzene (DVB) cross-linking which imparts physical stability to the resin polymer. The sulfonic acid functionality of the resin particles causes swelling in aqueous media. The resulting microporous resin particles can sorb water and nonionic solutes. The degree of molecular cross-linking with DVB influences the extent of sorption and prevents total dissolution of the porous resin. Because of ion repulsion and a high fixed acid chemical potential inside the resin microstructure, an electrolytic species, such as sulfuric acid in an acid/sugar mixture, for example, is effectively prevented from entering the porous resin. However, the nonionic sugar molecules are free to diffuse into the resin structure. Thus, electrolytes will pass through a packed resin bed faster than nonelectrolytes which are held up or delayed within the resin's microporous structure. In applying the instant invention to effect an acid separation similar to the separation used in the acid exchange system, supra, the resin used would be in its hydrogen form as opposed to the sodium form and, therefore, no ion exchange would occur in the system.
At the present time, ion exclusion technology is used for separation of ionic from nonionic or strongly ionic from weakly ionic solutes in polar media in certain analytical procedures, glycerin purification, and mixed acids separations applications (R. W. Wheaton and W. C. Bauman, Annals New York Academy of Sciences, 1953, Vol. 53, pp. 159-176). It differs from conventional ion exchange in that there is no net ion exchange between solute and resin. This eliminates the need for resin regeneration. Ion exclusion technology appears to have utility in separating ionic from nonionic species in aqueous solutions (D. W. Simpson and R. M. Wheaton, Chemical Engineering Progress, 1954, Vol. 50, No. 1, pp. 45-49). Basically, the ionic species are excluded from the fluid within the resin because of ionic repulsion within the resin particle micropore structure. This phenomena is explained by the Donnan exclusion principle, supra. Contrastingly, the nonionic species have no ionic repulsion with the resin and, therefore, penetrate the fluid within the porous resin to a greater degree. Thus, when a mixture of these two species is passed through a column of ion-exchange resin, the ionic component elutes first because it is excluded from the resin structure micropore volume. The nonionic species elutes after the ionic component because it has penetrated the resin micropore volume.
The physical and chemical characteristics of the resin are of vital importance to the design of an ion exclusion process. The total resin packed column volume can be thought of as to consist of three primary zones: 1) the macropore, also called void or interstitial volume, V.sub.o, which is the liquid volume between the resin particles, 2) the micropore volume, also known as occluded volume, V.sub.p, which is the liquid volume held within the resin particles, and 3) the solid resin network volume, V.sub.r, which is the actual structure of the resin (S. R. Nanguneri and R. D. Hester, Separation Sci. & Tech., 1990, Vol. 25, pp. 1829-1842). Due to the inherent ionic nature of the resin, an unequal distribution of ionic solute species exists between the micropore fluid (inside the resin) and macropore fluid (outside the resin) fluid phases. Thus, different resins with different pore volumes, ionic functionalities, and ionic charge density exhibit different separation characteristics with different solutes. The degree to which a solute species penetrates the micropore fluid of a resin is characterized by the distribution coefficient, K.sub.d. This distribution coefficient, sometimes referred to as the partition coefficient, of the i.sup.th solute species is defined as follows: EQU K.sub.di =C.sub.pi /C.sub.oi
where C.sub.pi is the concentration of the i.sup.th solute in the fluid located within the resin micropore volume and C.sub.oi is the concentration of the i.sup.th solute in the fluid outside the resin or in the macropore volume.
The value of K.sub.d varies with nature of the solute (ionic, nonionic), nature of resin (acidic, basic, resin cross-linkage density, particle mesh-size), solution composition, and temperature. The difference in K.sub.d values for different solutes is a good measure of the ease of species separation achievable by ion exclusion. The K.sub.d value determines the average time a species will elute from a packed resin bed. Each species flows through all the resin packed column interstitial fluid volume. However, the average fraction of resin micropore fluid volume penetrated by a species is proportional to the ratio of species concentration inside the resin to the concentration outside the resin. This ratio is the distribution coefficient, K.sub.di. Thus, the average fluid elution volume experienced by a species, V.sub.ei, is EQU V.sub.ei =V.sub.o +K.sub.di V.sub.pi
With the fluid volumetric flow rate, Q, known through a column packed with resin and the distribution coefficient of a solute, one can predict the average time, .theta..sub.i, of its appearance in the eluent. This time, prediction is accomplished through division of the species average elution volume by the fluid flow rate through the column. EQU .theta..sub.i =V.sub.ei /Q
Ionic species which do not penetrate or slightly penetrate into the resin micropore volume have distribution coefficients close to zero. Nonionic species which can penetrate the resin micropore volume have distribution coefficients greater than zero but less than one. If a chemical affinity exists between a species and the resin, then the distribution coefficient can exceed one.
Most of the ion exclusion chromatography used prior to the instant invention was of small scale and used only for analytical analysis as described in the following: (G. A. Harlow and D. H. Morman, Anal. Chem., 1964, Vol. 36, No. 13, pp. 2438-2442); (Y. Tikunga et al., J. Liquid Chrom., 1983, Vol. 6, No. 2, pp. 271-280); (K. Tanaka and T. Ishizuka, J. Liquid Chrom., 1979, Vol. 174, pp. 153-157); (K. Tanaka and J. S. Fritz, J. Liquid Chrom., 1986, Vol. 361, pp. 151-160); (K. Tanaka and J. S. Fritz, Anal. Chem., 1987, Vol. 59, No. 5, pp. 708-712); (K. Kihara et al J. Chrom., 1987, Vol. 410, pp. 103-110); and, (R. P. Neuman et al., Reactive Polymers, 1987, Vol. 5, pp. 55-61).
2. Description of the Prior Art
Ion exclusion, though widely used in analytical and pharmaceutical applications for many years, was not considered until recently for use in other than such applications due to the relatively low flux rates, small feed volumes, and weak electrolyte concentrations required to minimize dispersion and, thereby, provide for good species separation of the feedstock solution. Also, exacerbating the deleterious effects of dispersion caused by high flux rates, large feed volumes, and strong electrolyte concentrations was the dispersion caused by the presence of a so-called dead volume above the resin bed. Such dead volume resulted from shrinkage of the resin bed caused by the presence of a strong electrolyte such as sulfuric acid. Although identified as the primary factor contributing to dispersion, no successful means was devised until the discovery comprising the instant invention to deal with this phenomenon of dead volume caused by resin shrinkage.
The possibility of using strongly acidic cation exchange resins for the separation and recycle of acid from synthetic solutions of glucose and sulfuric acid has been investigated (R. P. Neuman et al., Reactive Polymers, 1987, Vol. 5, pp. 55-61). The work conducted at that time using Rohm and Hass Amberlite IR-118 resin in the hydrogen form and using small columns demonstrated the potential for this type of process chromatography. Note: Any reference made herein to materials and/or apparatus which are identified by means of trademarks, trade names, etc., are included solely for the convenience of the reader and are not intended as, or to be construed, an endorsement of said materials and/or apparatus. Although no actual hydrolyzates were used in the work reported by Neuman et al., the synthetic solution containing 7.7 percent H.sub.2 SO.sub.4 and 1.0 percent glucose showed separation of glucose from sulfuric acid at sample loading of 10 percent of the interstitial (column void) volume and at temperatures of 55.degree. C. and 68.degree. C. However, as noted by the authors, this work confirmed the potential for significant dispersion when operating even small ion exclusion systems.
The techniques revealed in the instant invention readily lend themselves to batch- or semi-continuous applications such as simulated moving bed (SMB) technology. SMB systems such as the Shanks merry-go-round have been applied in adsorption and ion exchange systems for many years. The Shanks system for leaching soda ash was introduced in England in 1841. The use of SMB or merry-go-round systems is quite common in the pharmaceutical industry as described in: (J. W. Chen et al., Ind. Eng. Chem. Process Des. Devel., 1972, Vol. 11, p. 430); for activated carbon adsorption in the chemical industry (H. J. Fornwalt and R. A. Hutchins, Chem. Eng., 1986, Vol. 73, No. 10, p. 155) and (M. J. Humenick, Jr., "Water and Wastewater Treatment," Calculations for Chemical and Physical Processes, Marcel Dekker, New York, chap. 6, 1977); for ion exchange in uranium purification (M. Streat, J. Sep. Process Technol., 1980, Vol. 1, No. 3, p. 10); and for waste water treatment with activated carbon (R. L. Culp et al., Handbook of Advanced Wastewater Treatment, 2nd ed., Van Nostrand- Reinhold, New York, chap. 6, 1978), and (C. T. Lawson and J. A. Fisher, AIChE Symp. Ser., 1974, Vol. 70, No. 136, p. 577), and (J. E. Parkhurst et al., J. Water Pollut. Control Fed., 1967, Vol. 39, p. 10). The primary advantages of SMB or similar systems in ion exclusion are the lower requirements (i.e., reductions of greater than 50 percent) for amounts of resin, water, and energy.